1. Field of the Invention
This invention relates to a process for producing a synthetic coking coal having a volatile matter content of about 25 to 45 wt % and a Gieseler fluidity of at least about 50,000 ddpm by thermal cracking of heavy hydrocarbons through the delayed coking process, which can be used as a substitute for natural coking coal. More particularly, this invention relates to a process for defoaming bubbles from the thermally cracked residue in a coking drum during thermal cracking as well as to a process for withdrawing said thermally cracked residue from the coking drum, cooling and solidifying whereby the thermally cracked residue is granulated, and further to a process for effectively recovering the heat of said thermally cracked residue. The present invention also includes the coking coal obtained by the above processes. The term "volatile matter content" as used herein is measured in accordance with ASTM D3175 and represents the percentage of gaseous products, exclusive of moisture vapor, in the coking coal. In addition, Gieseler fluidity is a relative measure of the plastic behavior of the coal as measured in accordance with ASTM D2639 using a Gieseler plastometer. The units of Gieseler fluidity, ddpm, are dial divisions per minute.
2. Description of the Prior Art
Processes have been proposed in U.S. Pat. Nos. 4,036,736 and 4,061,472 for heat treating heavy hydrocarbons to produce a substitute for coking coal suitable as the feedstock for coke production. In subsequent research it has been found that fluidity is a particularly important factor for any product capable of minimizing a shortage of coal feedstock which is likely to occur in Japan and that a thermally cracked residue having a Gieseler fluidity of at least about 50,000 ddpm serves best as the alternative to coking coal. A thermally cracked residue meeting this fluidity requirement and which can be processed in entirely the same manner as natural coking coal feedstock has a volatile matter content of about 25 to 45%, preferably about 30 to 45%, which is considerably higher than the 5 to 15% range of cokes produced by the conventional delayed coking process. Various difficulties occur if thermally cracked residues of such high volatile matter content are processed in entirely the same manner as the conventional delayed coking process. Withdrawal of the thermally cracked residue from the coking drum is particularly difficult and problems occur in each of the following steps of the delayed coking process:
(1) Cooling of the thermally cracked residue with water injected into the coking drum;
(2) Opening of the upper and lower flanges of the coking drum to the atmosphere; and
(3) Breaking of the thermally cracked residue with a jet-water cutting machine.
In step (1) above it is difficult to obtain a uniform dispersion of injected cooling water and the injection period as well as the cooling period are more than twice as long as in the conventional technique. When the flanges are opened to the atmosphere in step (2), as an inadequately cooled portion contact the air, inflammation is possible, and due to the plasticity of the thermally cracked residue obtained, breaking with a jet-water cutting machine in the step (3) is not efficient and requires a long time for achieving a desired result.
Furthermore, as thermal cracking of heavy hydrocarbons in a coking drum proceeds, bubbles vigorously form due to concurrently formed cracked oil vapor and cracked gas. Since part of these bubbles are still reactive, those withdrawn from the coking drum may obstruct the effluent line along which the cracked oil vapor and cracked gas from the top outlet of the coking drum are transferred to the downstream fractionator column, and deposits of the thermally cracked residue may form within the fractionator column, thus causing trouble in the operation of the process. In one technique used to prevent these drawbacks, a silicone defoaming agent is injected overhead into the coking drum. However, the thermally cracked residue formed within the coking drum according to the delayed coking process is coke, and bubbles form on or near the surface of the upper portion of the coke layer. In thermal cracking of heavy hydrocarbons as in this invention to produce synthetic coking coal, a major part of the thermally cracked residue in the coking drum is a viscous liquid, and a large quantity of tough bubbles are formed. As a result, the defoaming technique used in the past has been to use a great volume of a silicone defoaming agent or to use a large-scale coking drum in anticipation of the maximum formation of bubbles. As a further disadvantage, the silicone defoaming agent is decomposed in the coking drum and enters the cracked product. Thus, the use of a large amount of the defoaming agent is not desired in view of its effect on the quality of the product. On the other hand, increasing the volume of the coking drum by the volume of the bubbles rather than defoaming is not an economical method to take on an industrial scale.
As a result of extensive research on the physical properties at high temperatures as compared between the coke (volatile matter content: 50 to 15%) produced by the delayed coking process and the thermally cracked residue (volatile matter content: about 25 to 45%) which is to be treated by the process of this invention directed to solving the above problems, it has been found that the coke provided by the conventional delayed coking process accumulates in the coking drum as porous solid matter wiht no Gieseler fluidity at all, whereas the thermally cracked residue can be held in the form of a viscous liquid or slurry in the coking drum within a certain range of temperature. To be more specific, reference to FIG. 1 shows the relationship between the volatile matter content x (wt %) and the temperature of the thermally cracked residue T (.degree.C.). In the region (A) above the curve the relation T.ltoreq.0.293x.sup.2 -26.12x+790 holds and it has been found that a thermally cracked residue in the form of a viscous liquid or slurry that can be continuously withdrawn from the coking drum can be obtained. In addition, the withdrawn residue can be brought into contact with water for rapid cooling and solidification to form a granulated product having a particle size such that it can be immediately used, and the thermal energy of the cracked residue can be recovered in the form of steam.